GOALFEST:Arsenal were 4-0 down in the first half at the Madejski Stadium, but then Theo Walcott scored a hat-trick as they fought back to win 7-5 after extra-time

Arsenal’s Laurent Koscielny, center, scores against Reading in their League Cup match at the Madejski Stadium in Reading, England, on Tuesday.

Photo: Reuters

Arsenal staged a stunning League Cup fightback at Reading after trailing 4-0 in the first half on Tuesday, advancing to the quarter-finals with a 7-5 extra-time victory after Theo Walcott scored a hat-trick.

Walcott netted in stoppage-time in both halves and then in the last minute of extra-time as well in a game that always seemed to offer up a new twist.

“It was an unbelievable game. I’m absolutely knackered now,” Walcott said. “We’ve scored seven goals away from home — it was great going forward. Maybe defensively it wasn’t great at times, but we’re in the draw and that’s what matters.”

Arsenal’s performance in the first half was so poor that some Gunners fans left the stadium after the team conceded a fourth goal in the 37th minute, as Reading dominated against their Premier League rivals, but after Walcott pulled one back just before halftime, he took the game into extra-time with a last-gasp equalizer in the sixth minute of stoppage-time.

Substitute Olivier Giroud had netted the second for Arsenal and Laurent Koscielny, who earlier scored an own-goal, made it 4-3 in the 89th minute.

Marouane Chamakh put Arsenal ahead for the first time in extra-time with his first goal in almost a year, but Reading drew level again when Pavel Pogrebnyak made it 5-5 with four minutes to go.

Walcott then completed his hat-trick by slotting in a rebound from Andrei Arshavin’s effort and Chamakh lobbed goalkeeper Adam Federici in the closing exchanges to ensure Arsenal advanced.

“You always see new things in our game, that’s why it’s never boring,” Arsenal manager Arsene Wenger said. “It was a tennis match today. They had a fantastic start, we were not sharp, and it was 4-0 and could have been one or two more ... You cannot play for Arsenal and give up.”

Reading manager Brian McDermott was aggrieved that Walcott’s goal that took the game into extra-time came more than five minutes into stoppage-time when only four had been signaled by the fourth official.

“If it’s four minutes, you don’t mind another 30 seconds, but not another two minutes,” McDermott said. “I was waiting for him to blow his whistle, but he’ll have his reasons ... What happened tonight was extraordinary.”

On a thrilling night in English soccer’s second-tier knockout competition, three Premier League clubs — Southampton, Sunderland and Wigan Athletic — were beaten by lower-league opposition.

The biggest shock came at Wigan where fourth-tier club Bradford City won a penalty shootout 4-2 after the match ended 0-0.

Premier League strugglers Southampton exited after losing 3-0 to Leeds United of the second-tier Championship.

Goals from Michael Tonge and El-Hadji Diouf, as well as Luciano Becchio’s penalty kick, steered Leeds into the quarter-finals for the first time since 1996.

“We didn’t perform anywhere near what’s expected of a Southampton Football Club team,” said Southampton manager Nigel Adkins, who made 11 changes from the side that lost to Tottenham Hotspur on Sunday.

“Forty-eight hours ago we had a high-intensity game and I made the relevant changes. I take full responsibility,” he said. “We wanted to play tomorrow, but the powers that be wanted it to be tonight, but we gave the players who have not been in the team the chance to show what they could do and they did not take it.”